Current processes of producing fiberous ducts require sheet-forming of incoming feedstock, followed by fusing duct halves together, and trimming significant portions of the incoming stock off as un-recycled waste. Such techniques start with a certain thickness of incoming sheet-stock and effectively stretch the material into thinner sections during molding, which can severely limit both the shapes that can be formed and the uniformity of material distribution in the final product. Although surface profile corrugations can be formed into the parts for stiffening, it is generally not possible for these processes to incorporate ribs or thick sections since the forming process can only stretch and compress the material. Such products tend to be very flexible in nature, to the point of being floppy, which can require the addition of reinforcements, mounting tabs, and end-fittings so the products can be adequately assembled into a system. Thermoforming of this nature is generally regarded as a slow manufacturing process and is best suited for low-volume production.
The use of pulp-forming of cellulosic fiber packaging for consumer goods is well known. Thermoforming and matched-mold forming of either fibrous mat or extruded open-celled foam products is common practice for use with consumer goods. Blow molding of foamed polymers is becoming more prevalent, but the known technologies cannot currently produce sufficiently open-celled structures that are necessary to produce a truly acoustic duct used in an automobile. All known techniques suffer from sensitivity to stretch ratios, which makes wall thinning a challenge for deep draw areas.
Most applicable prior art is primarily defined by the potential for porosity and its importance for acoustic properties and, owing to the open structure of the material, increased thickness and improved thermal insulation. Such prior art primarily relates to matched-mold thermoforming of either partially or fully open-cell extruded foam sheets or nonwoven polymer fiber mat sheets. These techniques are limited in their capabilities in that a finite material must be stretched to match the contour of the mold wherein designs must be limited to restrict local stretch ratios and excessive thinning in deeply drawn areas. Another challenge with these techniques is that the material is generally utilized in rectangular blanks which, after forming, must be trimmed either before or after separate duct components are bonded together. In many cases, owing to the irregular nature of duct shapes, the inability to nest formed shapes into a mold layout limits part yield and results in a significant portion of the incoming material to be trimmed as scrap since the material is not directly recyclable back into the primary raw material stream and must be otherwise disposed of. Furthermore, the materials herein, being limited in thickness and stiffness, must frequently have expensive secondary applications of mounting tabs and duct ends for proper interfacing and assembly. All of the steps and waste inherent in the manufacture of such products, although while providing useful performance benefits, are too costly for broad market acceptance, thus they remain limited primarily to select premium vehicles; forcing lower cost vehicles to suffer with inefficient technologies.
WO2016/004522 A1 discloses a porous automotive HVAC duct composed of expanded polypropylene beads (EPP) which is formed in halves in a steam-chest process. However, EPP is not generally suitable for thin sections, such as for ducts, due to several shortcomings with the manner in which the material is fused only at their surfaces. While structures constructed from EPP are excellent in compression and generally applied as thick energy absorbers, they typically exhibit poor tensile and flexural properties. This material may serve well in permanent non-contact situations such as those behind instrument panels, but is less suited for floor ducts that may be exposed to high concentrated forces due to stepping loads, without considerable thickening or reinforcements. Thin EPP wall sections tend to break under conditions of tube-crush and component assembly forces. Molded EPP mounting tabs tend to be prone to breakage, and frequently must be made much thicker than is feasible for packaging, or must be added as injection, or otherwise molded, tabs in a secondary operation, further increasing system costs. Very thin wall sections, which are a frequent necessity in many tightly packaged situations, are ill-advised for EPP. Finally, expanded bead products, such as EPP and EPE, are specialty materials that cannot easily incorporate bio-materials or even recycled polymers.
What is lacking in the art is the use of cellulosic fiber technologies for the forming of automotive ducts and related items.